Image data compression

Abstract

Image data compression apparatus comprises a data compressor for receiving data representing sub-areas of an input image and for generating respective blocks of compressed data representing those sub-areas, each block of compressed data comprising at least low spatial frequency image data and higher spatial frequency image data; a data formatter for formatting predetermined groups of said blocks of compressed data into output data blocks, said data formatter having a predetermined data capacity; a detector for detecting whether a current group of said blocks of compressed data contains more data than said data capacity of said data formatter; and a block truncator, responsive to a detection that a current group of said blocks of compressed data contains more data than said data capacity of said data formatter, for truncating one or more of said blocks of compressed data by discarding at least said higher spatial frequency image data.

We claim: 
     
       1. Image data compression apparatus comprising: a data compressor for receiving data representing sub-areas of an input image and for generating respective blocks of compressed data representing those sub-areas, each block of compressed data comprising at least low spatial frequency image data and higher spatial frequency image data;   a data formatter for formatting predetermined groups of said blocks of compressed data into output data blocks, said data formatter having a predetermined data capacity;   a detector for detecting whether a current group of said blocks of compressed data contains more data than said data capacity of said data formatter; and   a block truncator, responsive to a detection that a current group of said blocks of compressed data contains more data than said data capacity of said data formatter, for truncating one or more of said blocks of compressed data by discarding at least said higher spatial frequency image data.   
     
     
       2. Apparatus according to claim 1, in which said output data blocks each have an output data block capacity. 
     
     
       3. Apparatus according to claim 2, in which said data capacity of said data formatter is greater than output data block capacity. 
     
     
       4. Apparatus according to claim 1, in which said data compressor is operable to generate blocks of compressed data respectively representing luminance and chrominance components of said sub-area of said image. 
     
     
       5. Apparatus according to claim 4, in which said low frequency image data of a luminance block of compressed data comprises a data value indicative of an average pixel luminance in said sub-area represented by that data block. 
     
     
       6. Apparatus according to claim 4, in which said low frequency image data of a chrominance block of compressed data comprises a data value indicative of an average pixel value of a chrominance component in said sub-area represented by that data block. 
     
     
       7. Apparatus according to claim 5, in which said block truncator is operable to truncate a luminance block by discarding said higher frequency image data but retaining said low frequency image data. 
     
     
       8. Apparatus according to claim 7, in which said block truncator is operable to include an end-of-block code in each truncated luminance block. 
     
     
       9. Apparatus according to claim 6, in which said block truncator is operable to truncate a chrominance block by discarding said low frequency image data and said higher frequency image data. 
     
     
       10. Apparatus according to claim 9, in which said block truncator is operable to include an end-of-block code in each truncated chrominance block. 
     
     
       11. Image data compression apparatus comprising: a data compressor for receiving data representing sub-areas of an input image and for generating respective blocks of compressed data representing those sub-areas, each block of compressed data comprising at least low spatial frequency image data and higher spatial frequency image data;   a data formatter for formatting predetermined groups of said blocks of compressed data into output data blocks, said data formatter having a predetermined data capacity;   a detector for detecting whether a current group of said blocks of compressed data contains more data than said data capacity of said data formatter; and   a block truncator, responsive to a detection that a current group of said blocks of compressed data contains more data than said data capacity of said data formatter, for truncating one or more of said blocks of compressed data by discarding said higher frequency image data and said low frequency image data of a block, and inserting a block error code.   
     
     
       12. A method of image data compression, said method comprising the steps of: receiving data representing sub-areas of an input image;   generating respective blocks of compressed data representing those sub-areas, each block of compressed data comprising at least low spatial frequency image data and higher spatial frequency image data;   formatting predetermined groups of said blocks of compressed data into output data blocks, according to a predetermined data formatting capacity;   detecting whether a current group of said blocks of compressed data contains more data than said data formatting capacity; and   in response to a detection that a current group of said blocks of compressed data contains more data than said data formatting capacity, truncating one or more of said blocks of compressed data by discarding at least said higher spatial frequency image data.   
     
     
       13. The method according to claim 12, wherein a block is truncated by discarding said higher frequency image data and said low frequency image data, and inserting a block error code. 
     
     
       14. The method according to claim 12, wherein said generating step generates blocks of compressed data respectively representing luminance and chrominance components of said sub-area of said image. 
     
     
       15. The method according to claim 14, wherein said low frequency image data of a luminance block of compressed data comprises a data value indicative of an average pixel luminance in said sub-area represented by that data block. 
     
     
       16. The method according to claim 14, wherein said low frequency image data of a chrominance block of compressed data comprises a data value indicative of an average pixel value of a chrominance component in said sub-area represented by that data block. 
     
     
       17. The method according to claim 15, wherein a luminance block is truncated by discarding said higher frequency image data but retaining said low frequency image data. 
     
     
       18. The method according to claim 17, further comprising including an end-of-block code in each truncated luminance block. 
     
     
       19. The method according to claim 16, wherein a chrominance block is truncated by discarding said low frequency image data and said higher frequency image data. 
     
     
       20. The method according to claim 19, further comprising including an end-of-block code in each truncated chrominance block.